Cellulose acetate propionate

ABSTRACT

Cellulose acetate propionate is a cellulose ester wherein hydroxyl groups of cellulose are substituted with acetyl and propionyl. Cellulose acetate propionate of the present invention has an amorphous index (Am) of 0.01 to 0.10. The present invention also provides cellulose acetate propionate having a degree of acetyl substitution (DSac) and a degree of propionyl substitution (DSpr) satisfying the formulas (I) to (IV). 
     
         2.0&lt;DSac≦2.95                                       (I) 
    
     
         0.05&lt;DSpr≦0.8                                       (II) 
    
     
         2.6&lt;DSac+DSpr≦3.0                                   (III) 
    
     
         1.9&lt;DSac-DSpr                                              (IV)

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of U.S. patent applicationSer. No. 08/902,888 filed on Jul. 30, 1997 and now U.S. Pat. No.5,856,468.

FIELD OF THE INVENTION

The present invention relates to cellulose acetate propionate.

BACKGROUND OF THE INVENTION

A cellulose acetate, particularly cellulose triacetate has been used toprepare various plastic products, such as a film or a fiber because thecellulose acetate is excellent in dimensional stability andheat-resistance. A cellulose acetate film is a representativephotogrpahic support. Further, the cellulose acetate film has an opticalisotropy. Accordingly, the film is also used in a liquid crystal displaydevice, which has recently extended its market. The cellulose acetatefilm is used as a protective film of a polarizing plate or a colorfilter in the liquid crystal display device.

A cellulose acetate product is prepared by using a solution (that iscalled "dope") or melt of cellulose acetate. For example, a celluloseacetate film is prepared according to a solvent cast method or a meltcast method. The solvent cast method comprises the steps of casting asolution of cellulose acetate in a solvent on a support, and evaporatingthe solvent to form a film. The melt cast method comprises the steps ofcasting molten cellulose acetate on a support under heating, and coolingit to form a film. The solvent cast method can form a highly flat film,compared with the melt cast method. Therefore, the solvent cast methodis generally employed to give a cellulose acetate film.

The solvent used in the solvent cast method must have functions not onlyof dissolving the cellulose acetate but also of forming an excellentfilm. In more detail, the viscosity and the polymer concentration of thesolution (dope) should be appropriately adjusted to form a flat planefilm having a uniform thickness. The dope also should have enoughstability. Further, the dope should easily be set to gel. Furthermore,the formed film should easily be peeled off the support. The mostappropriate solvent must be selected to satisfy these requirements.Moreover, the solvent should be so easily evaporated that the solventscarcely can remain in the film.

Various organic solvents have been proposed as the solvents of celluloseacetate. However, only methylene chloride satisfies all theabove-mentioned requirements. Accordingly, solvents other than methylenechloride have not been practically used.

However, the use of hydrocarbon halides such as methylene chloride hasrecently been restricted severely to protect the global environmentalconditions. Further, methylene chloride is apt to vaporize in theprocess for the preparation of the film, because it has a low boilingpoint (41° C.). Accordingly, methylene chloride may cause problems inthe working environment. Therefore, the process is conducted underclosed conditions. However, there is a technical limitation on sealingmethylene chloride in a closed system.

By the way, acetone and methyl acetate are widely used organic solvents.Acetone and methyl acetate have an appropriate boiling point (56° C. and57° C. respectively). The process of evaporating acetone or methylacetate does not need a large thermal energy. Further, acetone andmethyl acetate have few problems on the human body and the globalenvironmental conditions, compared with the organic chloride solvents.

However, cellulose acetate has a poor solubility in acetone or methylacetate. Cellulose triacetate having a degree of substitution of notmore than 2.80 (acetic acid content: 60.1%) is slightly soluble in, andmerely swelled in and acetone or methyl acetate.

C. J. Malm et al. report in Ind. Enig. Chem., 43 (1951) 688, thatsolvents for cellulose acetate are limited in number, compared withcellulose propionate or cellulose butyrate. Cellulose propionate orcellulose butyrate is soluble in a ketone or an ester, in whichcellulose acetate is insoluble. However, cellulose propionate andcellulose butyrate films are inferior to a cellulose acetate film in themechanical strength and the durability.

Cellulose acetate propionate and cellulose acetate butyrate arecommercially available. For example, a catalogue of Eastman (June 1994)shows various cellulose acetate propionates and cellulose acetatebutyrates, most of which are soluble in widely used organic solventssuch as acetone and methyl acetate. However, cellulose acetatepropionate and cellulose acetate butyrate films are still inferior to acellulose acetate film in the mechanical strength and the durability.The cellulose acetate propionate shown in the catalogue is not used in aprotective film or a photographic support (which requires a highmechanical strength), but is used in printing inks.

Japanese Patent Provisional Publication No. 6(1994)-501040 proposes amelt casting method in place of the solvent casting method having theproblems mentioned above. However, the melt casting method has anotherproblem that the melting point of cellulose triacetate is higher thanthe decomposition point. If cellulose triacetate, which has a highdegree of acetyl substitution, is heated, it would be decomposed beforemelted. The invention in the publication adjusts the degree of acetylsubstitution in the range of 1.9 to 2.6 to solve the problem ofdecomposition. The publication further discloses cellulose acetatepropionate having a degree of propionyl substitution in the range of 0to 0.9. In the Example B of the publication, cellulose acetatepropionate has the degree of acetyl substitution of 1.90 and the degreeof propionyl substitution of 0.71. In the Example C of the publication,cellulose acetate propionate has the degree of acetyl substitution of2.10 and the degree of propionyl substitution of 0.50.

SUMMARY OF THE INVENTION

The present inventors studied cellulose acetate propionate. Celluloseacetate (particularly cellulose triacetate) has a problem that solventsfor cellulose acetate are limited in number. The problem can be solvedby using cellulose propionate or cellulose acetate propionate in placeof cellulose acetate, as is described in the report of C. J. Malm et al.or the catalogue of Eastman. Cellulose propionate or cellulose acetatepropionate is inferior to cellulose triacetate in the physicalproperties. Cellulose acetate propionate disclosed in Japanese PatentProvisional Publication No. 6(1994)-501040 also has a problem in thephysical properties.

The present inventors have further studied cellulose acetate propionate,and found that the known cellulose acetate propionate has a low degreeof crystallinity. Therefore, a film made of the known cellulose acetatepropionate is soft and has a low mechanical strength.

An object of the present invention is to provide cellulose acetatepropionate that is soluble in various organic solvents, and has physicalproperties as the same as or better than those of cellulose triacetate.

The present invention provides cellulose acetate propionate having anamorphous index (Am) of 0.01 to 0.10 defined in the following formula:##EQU1## in which I(2θ=5°) and I(2θ=14.5°) mean X-ray scatteringintensities where Bragg angles (2θ) are 5° and 14.5° respectively, whichare obtained by forming a film of 100 μm thick from a cellulose acetatepropionate solution, treating the film at 200° C. for 60 minutes andmeasuring X-ray diffraction of the film; n means the number of peaksobserved in X-ray scattering intensity curve within the range of 5° to14.5° of Bragg angles (2θ) in the measurement of the film; and Pi meansX-ray scattering intensity at the peak number i in the measurement ofthe film.

The invention also provides cellulose acetate propionate having a degreeof acetyl substitution (DSac) and a degree of propionyl substitution(DSpr) satisfying the formulas (I) to (IV).

    2.0<DSac≦2.95                                       (I)

    0.05<DSpr≦0.8                                       (II)

    2.6<DSac+DSpr≦3.0                                   (III)

    1.9<DSac-DSpr                                              (IV)

The cellulose acetate propionate of the present invention is soluble inwidely used organic solvents other than hydrocarbon halides. Further,the cellulose acetate propionate of the invention has a high degree ofcrystallinity. Therefore, a product having excellent physical andoptical properties can be formed from the cellulose acetate propionatewithout use of hydrocarbon halide organic solvents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the X-ray diffraction of the film prepared inExample 1 wherein the ordinate means the X-ray scattering intensity(Kcps) and the abscissa means Bragg angle (2θ).

FIG. 2 is a graph showing the X-ray diffraction of the film prepared inComparison example 1 wherein the ordinate means the X-ray scatteringintensity (Kcps) and the abscissa means Bragg angle (2θ).

FIG. 3 is a graph showing the X-ray diffraction of the films prepared inExample 5 and Comparison examples 2 & 3 wherein the ordinate means theX-ray scattering intensity (Kcps) and the abscissa means Bragg angle(2θ).

FIG. 4 is a graph wherein the ordinate means the degree of propionylsubstitution (DSpr) and the abscissa means the degree of acetylsubstitution (DSac).

DETAILED DESCRIPTION OF THE INVENTION

[Definition of amorphous index]

The cellulose acetate propionate of the present invention ischaracterized in that the amorphous index (Am) is in the range of 0.01to 0.10. The amourphous index is more preferably in the range of 0.05 to0.10. The amorphous index (Am) is defined in the following formula.##EQU2##

In the formula, I(2θ=5°) and I(2θ=14.5°) mean X-ray scatteringintensities where Bragg angles (2θ) are 5° and 14.5° respectively. TheX-ray scattering intensities are obtained by forming a film of 100 μmthick from a cellulose acetate propionate solution, treating the film at200° C. for 60 minutes and measuring X-ray diffraction of the film. Inthe formula, n means the number of peaks observed in X-ray scatteringintensity curve within the range of 5° to 14.5° of Bragg angles (2θ) inthe measurement of the film. Further, Pi means X-ray scatteringintensity at the peak number i in the measurement of the film.

The measurement of the film is described in more detail.

First, cellulose acetate propionate is dissolved in a solvent to obtaina solution. A film of 100 μm thick is prepared according to a solventcasting method using the obtained cellulose acetate propionate solution.The film is then treated at 200° C. for 60 minutes to crystallizecellulose acetate propionate. The thermally treated film is pasted on analuminum sample folder. The X-ray diffraction intensity curve of thefilm is obtained by a symmetric reflection method. The X-ray generatoris a rotating anode type, and the X-ray source is Kα-ray of Cu. TheX-ray is passed thorough a monochromator to use a monochromatic X-ray.The operations are conducted at the tube voltage of 40 kV and the tubecurrent of 30 mA. The Bragg angle (2θ) of the measurement is in therange of 5° and 14.5°. The rotating speed of the goniometer is 2° (2θ)per minute. The obtained X-ray diffraction curve is not subjected tocorrections about the air scattering, a Lorentz polarizing factor or thelike. The fluctuations in the intensity curve are smoothed to obtain thescattering intensity.

The analysis of the X-ray diffraction intensity curve is described inmore detail in Examples 1 & 5 and Comparison examples 1 to 3 referringto FIGS. 1 to 3.

[Degrees of substitution]

The cellulose acetate propionate preferably has a degree of acetylsubstitution (DSac) and a degree of propionyl substitution (DSpr)satisfying the formulas (I) to (III).

    2.0<DSac≦2.95                                       (I)

    0.05<DSpr≦0.8                                       (II)

    2.6<DSac+DSpr≦3.0                                   (III)

The cellulose acetate propionate more preferably satisfies the formula(IV) as well as the formulas (I) to (III).

    1.9<DSac-DSpr                                              (IV)

The cellulose acetate propionate further preferably satisfies theformula (IIIa) in place of the formula (III).

    2.75<DSac+DSpr≦3.0                                  (IIIa)

Cellulose acetate propionate is a cellulose ester wherein some of threehydroxyl groups of a cellulose unit (glucose combined with β1-4glycoside bond) are substituted with acetyl and propionyl. The degree ofsubstitution means the ratio of substituted groups to the three hydroxylgroups of the cellulose unit. The degrees of substitution are calculatedfrom combined fatty acid contents (based on one cellulose unit), whichare measured according to ASTM, D-817-91 (Testing methods for celluloseacetate etc.).

The degree of substitution is described below referring to FIG. 4.

FIG. 4 is a graph wherein the ordinate means the degree of propionylsubstitution (DSpr) and the abscissa means the degree of acetylsubstitution (DSac).

The numbers 1 to 5, C2 and C3 shown in FIG. 4 mean the cellulose acetatepropionates used in Examples 1 to 5 and Comparison Examples 2 & 3respectively.

If a degree of acetyl substitution is lower than, or a degree ofpropionyl substitution is higher than the range of the formulas (I),(II) and (III) (if a plotted point is arranged in left or upper side ofthe area hatched in FIG. 4), an interaction of the cellulose estermolecule is so weak that the mechanical strength of the formed film(elastic modulus, folding endurance) is degraded. If a degree of acetylsubstitution is higher than, or a degree of propionyl substitution islower than the range of the formulas (I), (II) and (III) (if a plottedpoint is arranged in right or lower side of the area hatched in FIG. 4),solubilities of the cellulose ester in various organic solvents aredecreased.

Another acyl group or an ester group of an inorganic acid can be furtherattached to the cellulose acetate propionate so long as the degrees ofthe acetyl and propionyl substitutions included within theabove-mentioned range. An example of another acyl group is butyryl.Examples of inorganic acids include nitric acid, sulfuric acid andphosphoric acid.

The cellulose acetate propionate has a weight average degree ofpolymerization preferably in the range of 350 to 800, and morepreferably in the range of 370 to 600.

The cellulose acetate propionate has a number average molecular weightpreferably in the range of 70,000 to 230,000, more preferably in therange of 75,000 to 230,000, and most preferably in the range of 78,000to 120,000.

[Synthesis of cellulose acetate propionate]

Cellulose acetate propionate can be synthesized by using an acidanhydride or an acid chloride as an acylating agent. In the case that anacid anhydride is used as an acylating agent, an organic acid (e.g.,acetic acid) or methylene chloride is used as a reaction solvent, and anacidic catalyst such as sulfuric acid is used. In the case that an acidchloride is used as an acylating agent, a basic compound is used as acatalyst. According to the most conventional industrial synthesizingmethod, cellulose is esterified with organic acids (acetic acid,propionic acid) or anhydrides thereof (acetic anhydride, propionicanhydride) corresponding to acetyl and propionyl groups to synthesizecellulose acetate propionate.

The amounts of acetylating and propionylating agents are so adjustedthat the degrees of substitutions are included within theabove-mentioned range. The amount of the reaction solvent is preferablyin the range of 100 to 1,000 weight parts, more preferably in the rangeof 200 to 600 weight parts based on 100 weight parts of cellulose. Theamount of the acidic catalyst is preferably in the range of 0.1 to 20weight parts, and more preferably in the range of 0.4 to 10 weight partsbased on 100 weight parts of cellulose.

The reaction temperature is preferably in the range of 10 to 120° C.,and more preferably in the range of 20 to 80° C. Another acylating agent(e.g., butyrylating agent) or an esterifying agent (e.g., sulfuricesterifying agent) can be used in combination with the above-mentionedagents. If necessary, cellulose acetate propionate can be hydrolyzed(saponified) to adjust the degrees of substitutions after completing theacylating reaction.

After completing the reactions, the reaction mixture (cellulose esterdope) is separated according to a conventional method such assedimentation. The separated material is washed and dried to obtaincellulose acetate propionate.

[Cellulose acetate propionate solution]

Cellulose acetate propionate is dissolved in a solvent to prepare asolution.

An organic solvent is preferred to an inorganic solvent. The celluloseacetate propionate of the present invention is soluble in variousorganic solvents. The solubility is improved by the present inventionbecause a solubility parameter is changed by an effect of a randompolymerization.

A solution of the cellulose acetate propionate of the present inventioncan be prepared without use of hydrocarbon chlorides such as methylenechloride, dichloroethane and chloroform. The amount of hydrocarbonchloride is preferably not more than 5 weight %, and more preferably notmore than 3 weight % of the organic solvent. The organic solvent furtherpreferably substantially does not contain hydrocarbon chloride, and mostpreferably completely does not contain hydrocarbon chloride.

Examples of the organic solvents include ketones (e.g., acetone, methylethyl ketone), nitro compounds (e.g., nitromethane, nitroethane), esters(e.g., methyl formate, ethyl formate, methyl acetate, ethyl acetate),amides (e.g., N,N-dimethylformamide, N-methylpyrrolidone), cyclic ethers(e.g., tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane),cellosolves (e.g., methylcellosolve, ethylcellosolve, methylcellosolveacetate, ethylcellosolve acetate), nitriles (e.g., acetonitrile) andsulfoxides (e.g., dimethylsulfoxide). The cellulose acetate propionateof the present invention has a high solubility particularly in ketonessuch as acetone and esters such as methyl acetate, compared withcellulose triacetate. Two or more organic solvents can be used incombination. A lower alcohol having 1 to 4 carbon atoms (e.g., methanol,n-butanol) or cyclohexanone can be used in combination with theabove-mentioned organic solvent.

A cellulose ester solution can be prepared in the same manner as in aconventional solvent casting method. The process and apparatus for thesolvent casting method can be used in the preparation of the celluloseacetate propionate solution.

A solution of a relatively low concentration can be prepared by stirringa cellulose ester in a solvent at the room temperature. A solution of arelatively high concentration is preferably prepared by stirring theester in the solvent while heating under a pressure. In more detail,cellulose acetate propionate and a solvent are placed in apressure-resistant vessel, and sealed. The mixture is then stirred at atemperature of higher than the boiling point (at atmospheric pressure)of the solvent. The pressure is so adjusted that the solvent is notboiled in the vessel. The heating temperature is usually not lower than60° C., and preferably in the range of 80 to 110° C.

After coarsely mixing components of the solution, the mixture can beplaced in the vessel. Alternatively, each of the components can stepwisebe introduced into the vessel in the order. A stirring device ispreferably attached to the vessel. The pressure in the vessel can beincreased by injecting an inactive gas such as nitrogen gas into thevessel. The pressure can also be increased by evaporation of the heatedsolvent. Further, the components can be introduced into the vessel bypressure after sealing the vessel.

A heating means is preferably arranged outside the vessel. For example,the vessel can be jacketed with a heating means. Further, a plate heatercan be attached outside the vessel. A heated liquid is circulated intube contained in the plate heater to heat the vessel.

A stirring wing is preferably arranged in the vessel to stir the mixtureof cellulose acetate propionate in a solvent. The length of the wing ispreferably adjusted to almost reach the inside wall of the vessel. Ascratching wing is preferably attached at the end of the stirring wingto scratch a liquid membrane formed along the inside wall of the vessel.

Meters such as a thermometer and a manometer can be attached to thevessel. The components are dissolved in a solvent in the vessel. Aftercooling the prepared solution (dope), the solution is discharged fromthe vessel. Alternatively, the solution can be cooled after the solutionis discharged from the vessel.

Even if cellulose acetate propionate is not sufficiently dissolved in anorganic solvent according to the above-described conventional method(conducted at room temperature or an elevated temperature), celluloseacetate propionate can be dissolved in the solvent according to acooling dissolution method. The cooling dissolution method comprises thesteps of: mixing cellulose acetate propionate in an organic solvent;cooling the mixture to -110° to 20° C.; warming the cooled mixture to 0°to 120° C. to dissolved the cellulose acetate propionate in the organicsolvent.

At the first stage of cooling dissolution method, cellulose acetatepropionate is gradually added to an organic solvent while stirring atroom temperature. Cellulose acetate propionate is swelled with theorganic solvent, but is not dissolved at this stage. The amount ofcellulose acetate propionate is in the range of 10 to 40 wt. %, based onthe amount of the mixture. The amount is preferably in the range of 10to 30 wt. %. The other optional additives (described below) may be addedto the organic solvent.

At the next stage, the mixture is cooled to a temperature of -110° to20° C. The mixture can be cooled in a dry ice/methanol bath (-75° C.) orin a cooled diethylene glycol solution (-30° to -20° C.). At the coolingstage, the solvent permeates into the swelled cellulose acetatepropionate (white clouds) to expel the air as bubbles from the mixture.The mixture turns colorless transparent.

Subsequently, the mixture is warmed to a temperature of 0° to 120° C. todissolve the cellulose acetate propionate in the organic solvent. Themixture can be warmed by keeping it at room temperature. The mixture canalso be warmed on a bath. Thus a dope is formed as a uniform solution.If cellulose acetate propionate is not sufficiently dissolved, thecooling and warming steps can be repeated. The dope is observed witheyes to determine whether cellulose acetate propionate is sufficientlydissolved or not.

A sealed vessel is preferably used in the cooling dissolution method toprevent contamination of water, which is caused by dew condensation atthe cooling step. The time for the cooling and warming steps can beshortened by conducting the cooling step under a high pressure andconducting the warming step under a low pressure. A pressure vessel ispreferably used under a high or low pressure.

The cooling dissolution method is described in U.S. Pat. No. 5,663,310(Shimoda et al.) and U.S. Pat. No. 5,705,632 (Shimoda et al.).

The concentration of the solution is determined according to use of thesolution. The solution contains cellulose acetate propionate usually inthe range of 5 to 50 wt. %, and preferably in the range of 10 to 40 wt.%.

In the case that the solution is used to prepare a cellulose acetatepropionate film, the solution has a viscosity preferably in the range of10,000 to 1,000,000 cP.

Other additives (e.g., a plasticizer, inorganic powder, a stabilizer, acoloring agent, an antioxidizing agent, an antistatic agent, anultraviolet absorbent, a flame retarder) can be added to the solutionaccording to use of the solution.

A cellulose ester product usually contains a plasticizer to improve thephysical property or to increase the drying speed. A phosphoric ester ora carboxylic ester is usually used as the plasticizer. Examples of thephosphoric esters include triphenyl phosphate and tricresyl phosphate.Representative carboxylic ester plasticizers are phthalic esters andcitric esters. Examples of the phthalic esters include dimethylphthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate anddiethylhexyl phthalate. Examples of the citric esters includeacetyltriethyl citrate and acetyl triethyl citrate. Examples of theother carboxylic esters include butyl oleate, methyl acetyl ricinoleate,dibutyl sebacate, triacetin, triethylene glycol diacetate, triethyleneglycol dipropionate and various trimellitic esters. Two or moreplasticizers can be used in combination.

The amount of the plasticizer is usually in the range of 0.1 to 40 wt. %based on the amount of cellulose acetate propionate.

Examples of inorganic powders include kaolin, talc, diatomite, quartz,calcium carbonate, barium sulfate, titanium dioxide and alumina.

Examples of stabilizers include salts of alkaline earth metals (e.g.,calcium, magnesium) and triphenyl phosphate.

[Cellulose acetate propionate film]

A cellulose acetate propionate film can be prepared according to asolvent casting method by using the cellulose acetate propionatesolution. The solvent casting method is described in U.S. Pat. Nos.2,336,310, 2,367,603, 2,492,077, 2,492,078, 2,607,704, 2,739,069,2,739,070, British Patent Nos. 640,731, 736,892, Japanese PatentPublication Nos. 45(1970)-4554, 49(1974)-5614, Japanese PatentProvisional Publication Nos. 60(1985)-176834, 60(1985)-203430 and62(1987)-115035.

According to a conventional solvent casting method, a prepared celluloseester solution (dope) is cast on a support (e.g., drum, band), anddried. The formed film is peeled from the support. The surface of thesupport is preferably polished to give a mirror plane.

The film is dried usually at a temperature in the range of 20 to 250° C.(preferably 30 to 200° C.) at atmospheric pressure or under a reducedpressure.

The thickness of the film is determined depending on use of the film.The thickness is usually in the range of 0.1 to 250 μm. An optical thinfilm for the protection of IC mask usually has a thickness in the rangeof 0.1 to 3 μm. A lapping film usually has a thickness in the range of10 to 50 μm. Further, a photographic or optical film usually has athickness in the range of 50 to 250 μm.

[Other cellulose acetate propionate products]

cellulose ester fibers can be prepared from the cellulose acetatepropionate of the present invention. The cellulose ester fibers can beprepared according to a conventional method, for example by spinningfibers from the solution (dope) and removing the solvent from thefibers. The process of drying the fibers can be conducted in the samemanner as in the above-mentioned process of drying the film. Thecellulose ester fibers have a thickness preferably in the range of 1 to16 deniers, more preferably in the range of 1 to 10 deniers, and mostpreferably in the range of 2 to 8 deniers. There is no specificlimitation with respect to the sectional shapes of the fibers. Examplesof the sectional shapes include a round shape, an oval shape, anirregular shape (e.g., the shapes of the letters Y, X, I or R) and ahollow shape.

The other various cellulose ester products can be prepared by using thecellulose acetate propionate of the present invention. The products canbe prepared by using not only a solution (dope) or a melt of thecellulose acetate propionate but also cellulose acetate propionateparticles. According to an extrusion or injection molding method, aproduct can be prepared from the cellulose acetate propionate particles.

In the preparation of the product, other cellulose esters can be used incombination with the cellulose acetate propionate of the presentinvention. Examples of the other cellulose esters include organic acidesters (e.g., cellulose acetate, cellulose propionate, cellulosebutyrate) and inorganic acid esters (e.g., cellulose nitrate, cellulosesulfate, cellulose phosphate).

Products prepared by using the cellulose acetate propionate of thepresent invention are excellent in the mechanical strength and theoptical property. The products are particularly excellent in the dynamicviscoelasticity. For example, the film of the present invention has acomplex modulus or a storage modulus usually in the range of 2.8×10⁹ to8×10⁹ (preferably in the range of 3×10⁹ to 5×10⁹). Further, the losstangent (tan δ) is usually not more than 0.034 (preferably in the rangeof 0.02 to 0.034).

Because of the above-mentioned excellent physical properties, thecellulose acetate propionate of the present invention can advantageouslybe used to prepare a photogrpahic film or an optical film (e.g., apolarizing film), which severely requires excellent physical properties.

EXAMPLE 1

To 300 g of cellulose, 896 g of acetic acid and 203 g of propionic acidwere added. The mixture was stirred at 54° C. for 30 minutes. Aftercooling the mixture, 419 g of acetic anhydride, 622 g of propionicanhydride, 10.6 g of sulfuric acid and 6.3 g of propionic acid (each ofwhich was precooled at about -20° C.) were added to the mixture toinitiate an esterifying reaction. The reaction was so controlled thatthe reaction temperature was not higher than 40° C. After theesterifying reaction was continued for 155 minutes, a mixture (reactionstopping agent) of 295 g of acetic acid and 98.5 g of water were addedto the reaction mixture for 20 minutes to hydrolyze extra anhydrides. Tothe reaction mixture, 886 g of acetic acid and 295 g of water were addedwhile keeping the temperature of the mixture at 60° C. After 1 hour, anaqueous solution of 17.0 g of magnesium acetate was added to the mixtureto neutralize sulfuric acid in the mixture.

The obtained cellulose acetate propionate had the degree of acetylsubstitution (DSac) of 2.31 and the degree of propionyl substitution(DSpr) of 0.61. The weight average degree of polymerization was 560, andthe number average molecular weight was 88,900.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

A 10 wt. % solution of the cellulose acetate propionate in chloroformwas cast on a glass plate, and air-dried for 1 day. The formed film(thickness: 100 μm) was dried under vacuum at 80° C. for 4 hours. Thefilm was heated at 200° C. for 60 minutes, and was subjected to theX-ray diffraction. The obtained X-ray diffraction is shown in FIG. 1. InFIG. 1, the ordinate means the X-ray scattering intensity (Kcps) and theabscissa means Bragg angle (2θ).

As is shown in FIG. 1, three peaks (P1, P2, P3) were observed within theBragg angle (2θ) range of 5 to 14.5°. The amorphous index (Am) wascalculated according to the formula.

    Am=0.5×{I(2θ=5°)+I(2θ=14.5°)}÷(P1+P2+P3)=0.08

Further, the sample film (thickness: 100 μm) was cut into pieces of 2 mmwidth and 35 mm length. Both ends of the piece was fixed, and vibrationwas forced to the piece to measure distortion. The complex modulus, thestorage modulus and the loss tangent (tan δ) were obtained from therelation between the stress and the distortion. The measurement wasconducted by using a solid-state automatic viscoelasticity meter(RSA-II, Reometrix). At the measure ment, the frequency was 10 Hz at 26°C.

The results are set forth in Table 1.

COMPARISON EXAMPLE 1

A commercially available cellulose acetate propionate (Cellidore, Bayer,DSac: 0.32, DSpr: 2.32) was dissolved in chloroform to prepare a 10 wt.% solution. The solution was cast on a glass plate, and air-dried for 1day. The formed film (thickness: 100 μm) was dried under vacuum at 80°C. for 4 hours. The film was heated at 200° C. for 60 minutes, and wassubjected to the X-ray diffraction. The obtained X-ray diffraction isshown in FIG. 2. In FIG. 2, the ordinate means the X-ray scatteringintensity (Kcps) and the abscissa means Bragg angle (2θ).

As is shown in FIG. 2, one peak (P1) was observed within the Bragg angle(2θ) range of 5 to 14.5°. The amorphous index (Am) was calculatedaccording to the formula.

    Am=0.5×{I(2θ=5°)+I(2θ=14.5°)}÷P1=0.50

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

EXAMPLE 2

To 303 g of cellulose, 908 g of acetic acid and 200 g of propionic acidwere added. The mixture was stirred at 54° C. for 30 minutes. Aftercooling the mixture, 701 g of acetic anhydride, 276 g of propionicanhydride, 10.6 g of sulfuric acid and 6.3 g of propionic acid (each ofwhich was precooled at about -20° C.) were added to the mixture toinitiate an esterifying reaction. The reaction was so controlled thatthe reaction temperature was not higher than 40° C. After theesterifying reaction was continued for 150 minutes, a mixture (reactionstopping agent) of 295 g of acetic acid and 98.5 g of water were addedto the reaction mixture for 20 minutes to hydrolyze extra anhydrides. Tothe reaction mixture, 886 g of acetic acid and 295 g of water were addedwhile keeping the temperature of the mixture at 60° C. After 1 hour, anaqueous solution of 17.0 g of magnesium acetate was added to the mixtureto neutralize sulfuric acid in the mixture.

The obtained cellulose acetate propionate had the degree of acetylsubstitution (DSac) of 2.60 and the degree of propionyl substitution(DSpr) of 0.30. The weight average degree of polymerization was 520, andthe number average molecular weight was 75,800.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

EXAMPLE 3

To 303 g of cellulose, 881 g of acetic acid and 203 g of propionic acidwere added. The mixture was stirred at 54° C. for 30 minutes. Aftercooling the mixture, 605 g of acetic anhydride, 400 g of propionicanhydride, 10.6 g of sulfuric acid and 6.3 g of propionic acid (each ofwhich was precooled at about -20° C.) were added to the mixture toinitiate an esterifying reaction. The reaction was so controlled thatthe reaction temperature was not higher than 40° C. After theesterifying reaction was continued for 140 minutes, a mixture (reactionstopping agent) of 295 g of acetic acid and 98.5 g of water were addedto the reaction mixture for 20 minutes to hydrolyze extra anhydrides. Tothe reaction mixture, 886 g of acetic acid and 295 g of water were addedwhile keeping the temperature of the mixture at 80° C. After 40 minutes,an aqueous solution of 17.0 g of magnesium acetate was added to themixture to neutralize sulfuric acid in the mixture.

The obtained cellulose acetate propionate had the degree of acetylsubstitution (DSac) of 2.38 and the degree of propionyl substitution(DSpr) of 0.39. The weight average degree of polymerization was 541, andthe number average molecular weight was 83,800.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

EXAMPLE 4

To 301 g of cellulose, 753 g of acetic acid and 331 g of propionic acidwere added. The mixture was stirred at 54° C. for 30 minutes. Aftercooling the mixture, 919 g of acetic anhydride, 10.6 g of sulfuric acidand 6.3 g of propionic acid (each of which was precooled at about -20°C.) were added to the mixture to initiate an esterifying reaction. Thereaction was so controlled that the reaction temperature was not higherthan 40° C. After the esterifying reaction was continued for 140minutes, a mixture (reaction stopping agent) of 295 g of acetic acid and98.5 g of water were added to the reaction mixture for 20 minutes tohydrolyze extra anhydrides. To the reaction mixture, 886 g of aceticacid and 295 g of water were added while keeping the temperature of themixture at 80° C. After 1 hour, an aqueous solution of 17.0 g ofmagnesium acetate was added to the mixture to neutralize sulfuric acidin the mixture.

The obtained cellulose acetate propionate had the degree of acetylsubstitution (DSac) of 2.71 and the degree of propionyl substitution(DSpr) of 0.17. The weight average degree of polymerization was 602, andthe number average molecular weight was 83,400.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

EXAMPLE 5

Cellulose acetate propionate having the degree of acetyl substitution(DSac) of 2.42 and the degree of propionyl substitution (DSpr) of 0.56was synthesized.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

A 10 wt. % solution of the cellulose acetate propionate in chloroformwas cast on a glass plate, and air-dried for 1 day. The formed film(thickness: 100 μm) was dried under vacuum at 80° C. for 4 hours. Thefilm was heated at 200° C. for 60 minutes, and was subjected to theX-ray diffraction. The obtained X-ray diffraction is shown in FIG. 3. InFIG. 3, the ordinate means the X-ray scattering intensity (Kcps) and theabscissa means Bragg angle (2θ).

As is shown in FIG. 3, three peaks were observed within the Bragg angle(2θ) range of 5 to 14.5°. The amorphous index (Am) was calculatedaccording to the formula.

    Am=0.5×{I(2θ=5°)+I(2θ=14.5°)}÷(P1+P2+P3)=0.07

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

COMPARISON EXAMPLE 1

Cellulose acetate propionate having the degree of acetyl substitution(DSac) of 2.08 and the degree of propionyl substitution (DSpr) of 0.56was synthesized.

The cellulose acetate propionate was mixed with acetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

A 10 wt. % solution of the cellulose acetate propionate in chloroformwas cast on a glass plate, and air-dried for 1 day. The formed film(thickness: 100 μm) was dried under vacuum at 80° C. for 4 hours. Thefilm was heated at 200° C for 60 minutes, and was subjected to the X-raydiffraction. The obtained X-ray diffraction is shown in FIG. 3. In FIG.3, the ordinate means the X-ray scattering intensity (Kcps) and theabscissa means Bragg angle (2θ).

As is shown in FIG. 3, three peaks were observed within the Bragg angle(2θ) range of 5 to 14.5°. The amorphous index (Am) was calculatedaccording to the formula.

    Am=0.5×{I(2θ=5°)+I(2θ=14.5°)}÷(P1+P2+P3)=0.12

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

COMPARISON EXAMPLE 2

Cellulose acetate propionate having the degree of acetyl substitution(DSac) of 1.77 and the degree of propionyl substitution (DSpr) of 0.51was synthesized.

The cellulose acetate propionate was mixed with cetone, methyl acetateor chloroform at the room temperature. The cellulose acetate propionatewas dissolved in each of the organic solvents to obtain a solution of 10wt. % or more.

A 10 wt. % solution of the cellulose acetate propionate in chloroformwas cast on a glass plate, and air-dried for 1 day. The formed film(thickness: 100 μm) was dried under vacuum at 80° C. for 4 hours. Thefilm was heated at 200° C. for 60 minutes, and was subjected to theX-ray diffraction. The obtained X-ray diffraction is shown in FIG. 3. InFIG. 3, the ordinate means the X-ray scattering intensity (Kcps) and theabscissa means Bragg angle (2θ).

The amorphous index (Am) was calculated according to the formula.

    Am=0.5×{I(2θ=5°)+I(2θ=14.5°)}÷ΣPi=0.12

Further, the complex modulus, the storage modulus and the loss tangent(tan δ) were obtained in the same manner as in Example 1.

The results are set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                               Degree of  Amor-    Elastic modulus                                                                           Los                                      Sample substitution phous (× 10.sup.-9 Pa) tangent                    No.    DSac    DSpr   index  Complex                                                                              Storage                                                                              (tan δ)                      ______________________________________                                        Ex. 1  2.31    0.61   0.08   3.23   3.23   0.031                                Comp. 1 0.32 2.32 0.50 2.56 2.56 0.036                                        Ex. 2 2.61 0.30 0.07 3.26 3.26 0.029                                          Ex. 3 2.38 0.39 0.10 3.24 3.24 0.031                                          Ex. 4 2.71 0.17 0.08 3.94 3.94 0.030                                          Ex. 5 2.42 0.56 0.07 4.06, 4.05 0.029                                         Comp. 2 2.08 0.56 0.12 3.06 3.05 0.037                                        Comp. 3 1.77 0.51 0.74 2.98 2.97 0.042                                      ______________________________________                                    

As is evident from the results shown in Table 1, the films preparedaccording to the present invention have a high elasticity and a low losstangent (tan δ) and compared with the film of the comparative sample.The results mean that the films prepared according to the invention arehard, and have excellent mechanical properties.

We claim:
 1. Cellulose acetate propionate having an amorphous index (Am)of 0.01 to 0.10 defined in the following formula: ##EQU3## in whichI(2θ=5°) and I(2θ=14.5°) mean X-ray scattering intensities where Braggangles (2θ) are 5° and 14.5° respectively, which are obtained by forminga film of 100 μm thick from a cellulose acetate propionate solution,treating the film at 200° C. for 60 minutes and measuring X-raydiffraction of the film; n means the number of peaks observed in X-rayscattering intensity curve within the range of 5° to 14.5° of Braggangles (2θ) in the measurement of the film; and Pi means X-rayscattering intensity at the peak number i in the measurement of thefilm.
 2. The cellulose acetate propionate as claimed in claim 1, whereinthe amorphous index (Am) is in the range of 0.05 to 0.10.
 3. Thecellulose acetate propionate as claimed in claim 1, wherein thecellulose acetate propionate has a degree of acetyl substitution (DSac)and a degree of propionyl substitution (DSpr) satisfying the formulas(I) to (III).

    2.0<DSac≦2.95                                       (I)

    0.05<DSpr≦0.8                                       (II)

    2.6<DSac+DSpr≦3.0                                   (III).


4. 4. The cellulose acetate propionate as claimed in claim 1, whereinthe cellulose acetate propionate has a weight average degree ofpolymerization in the range of 350 to
 800. 5. The cellulose acetatepropionate as claimed in claim 1, wherein the cellulose acetatepropionate has a number average molecular weight in the range of 70,000to 230,000.
 6. The cellulose acetate propionate as claimed in claim 1,wherein the cellulose acetate propionate has a degree of acetylsubstitution (DSac) and a degree of propionyl substitution (DSpr)satisfying the formula (IIIa).

    2.75<DSac+DSpr≦3.0                                  (IIIa)


7. Cellulose acetate propionate having a degree of acetyl substitution(DSac) and a degree of propionyl substitution (DSpr) satisfying theformulas (I) to (IV).

    2.0<DSac≦2.95                                       (I)

    0.05<DSpr≦0.8                                       (II)

    2.6<DSac+DSpr≦3.0                                   (III)

    1.9<DSac-DSpr                                              (IV)


8. The cellulose acetate propionate as claimed in claim 7, wherein thecellulose acetate propionate having a degree of acetyl substitution(DSac) and a degree of propionyl substitution (DSpr) satisfying theformula (IIIa).

    2.75<DSac+DSpr≦3.0                                  (IIIa)